Multiple power supplies providing enhanced power efficiency

ABSTRACT

Method and computer program product for supplying power in a computing system, and computer program product implementing the method. The method comprises monitoring power consumption of the computing system, supplying power to the computing system using only a first power supply over a first range of power consumption, and supplying power to the computing system using a combination of the first power supply and a second power supply over a second range of power consumption. The first power supply provides greater efficiency than the combination of the first and second power supplies over the first lower range of power consumption, the combination of the first and second power supplies provides greater efficiency than the first power supply over the second higher range of power consumption.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to supplying power to a computer system,and more specifically to methods of improving the power efficiency of acomputer system.

2. Background of the Related Art

The amount of power consumed by the equipment in a modern datacentersrepresents a significant expense, even in relation to the cost of theequipment itself. A modern datacenter may include as many as tens ofhundreds of high-power, rack-mounted servers disposed in high-densityenclosures. Management of the power consumed by this equipment is animportant consideration in the design and operation of any datacenter.

Attempts to optimize power consumption may be multifaceted. For example,power consumption may be reduced by improving datacenter layout andcomponent selection. Furthermore, the design of individual componentscan affect the overall efficiency of a server or other device. Powermanagement solutions may include both hardware and software aspectsaimed at improving various aspects of power utilization, allocation, andload scheduling.

BRIEF SUMMARY OF THE INVENTION

One embodiment of the present invention provides a method for supplyingpower in a computing system. The method comprises monitoring powerconsumption of the computing system, supplying power to the computingsystem using only a first power supply over a first range of powerconsumption, and supplying power to the computing system using acombination of the first power supply and a second power supply over asecond range of power consumption. The first power supply providesgreater efficiency than the combination of the first and second powersupplies over the first lower range of power consumption, and thecombination of the first and second power supplies provides greaterefficiency than the first power supply over the second higher range ofpower consumption. In this embodiment, until this optimal powerconsumption is reached (i.e., until the first range of power consumptionhas been exceeded), the second power supply remains completely off, notrequiring even stand-by power.

Another embodiment of the invention provides a computer program productincluding computer usable program code embodied on a computer usablestorage medium for managing the supply of power in a computer system.The computer program product is preferably executed by a managemententity, and comprises computer usable program code for monitoring powerconsumption of the computing system, computer usable program code forinstructing only a first power supply to supply power to the computingsystem over a first range of power consumption, and computer usableprogram code for instructing a combination of the first power supply anda second power supply to supply power to the computing system over asecond range of power consumption. As with the foregoing method, thefirst power supply provides greater efficiency than the combination ofthe first and second power supplies over the first lower range of powerconsumption, the combination of the first and second power suppliesprovides greater efficiency than the first power supply over the secondhigher range of power consumption.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of a single server blade of a computer systemthat is being monitored by a management entity in order to efficientlysupply power to the computer system.

FIG. 2 is a graph of power efficiency as a function of power consumptionincluding a power efficiency curve for a single power supply and a powerefficiency curve for a combination of two synchronous power supplies.

FIG. 3A is a diagram showing supply of power to a computer system usinga single power supply.

FIG. 3B is a diagram showing supply of power to a computer system usinga combination of power supplies operating synchronously.

FIG. 4 is a flowchart of a method of supplying power to a computersystem.

FIG. 5 is a graph of power savings as a function of power consumed in arange near the cross-over in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the present invention provides a method for supplyingpower in a computing system, and computer program product implementingthe method. The method comprises monitoring power consumption of thecomputing system, supplying power to the computing system using only afirst power supply over a first range of power consumption, andsupplying power to the computing system using a combination of the firstpower supply and a second power supply over a second range of powerconsumption. The first power supply provides greater efficiency than thecombination of the first and second power supplies over the first lowerrange of power consumption, the combination of the first and secondpower supplies provides greater efficiency than the first power supplyover the second higher range of power consumption. The reason that thepower supply efficiency decreases at high power output is because thefield effect transistors used in regulating power supply current have afinite resistance (R). The loss in the efficiency of the powerconsumption is related to the square of the current drawn, e.g.P_(loss)=I² output times R_(FET). One can see by these equations thatreducing the output current at higher current consumptions (higher poweroutputs) has a significant power savings.

In another embodiment, the method further comprises switching from thefirst power supply to the combination of the first and second powersupplies in response to the power consumption reaching a first powerconsumption setpoint, and switching from the combination of the firstand second power supplies to the first power supply in response to thepower consumption reaching a second power consumption setpoint, whereinthe first power consumption setpoint is greater than the second powerconsumption setpoint. The first power consumption setpoint is preferablygreater than the power consumption corresponding to the cross-overefficiency between the first power supply and the combination of thefirst and second power supplies. It is also preferable that the secondpower consumption setpoint is less than the power consumptioncorresponding to the cross-over efficiency between the first powersupply and the combination of the first and second power supplies. Thispreferred method provides a hysteresis function to ensure that there isno power supply oscillation near the first power consumption setpoint.Optionally, the first power consumption setpoint, the second powerconsumption setpoint, or both may have a predetermined value, forexample as a result of empirical testing of power supply efficiencies.

In a further embodiment, the computer system includes a plurality ofcomputer servers, and the power consumption is that amount of powerconsumed by the plurality of computer servers. Preferably, a managemententity monitors the power consumption of the computer system byreceiving power consumption data from each of the plurality of computerservers. This may be accomplished where each of the plurality ofcomputer servers includes a baseboard management controller thatprovides server power consumption data to the management entity.

FIG. 1 is a block diagram of a plurality of server blades 20 within acomputer system 10 that is being monitored by a management entity 40 inorder to efficiently supply power from the power supplies 50A, 50B tothe computer system. Each server blade 20 includes a “Super I/O”controller 22 and a baseboard management controller (BMC) 24 whichreceives power monitoring data 26 from other components (not shown)within the server blade 20 and has access to vital product data (VPD) 28about the server blade 20 and its components. The BMC 24 communicateswith the management entity 40 over a communication line, such as aserial port compatible with the RS-485 standard. Furthermore, themanagement entity 40 communicates with a voltage regulator controllerfor each of the power supplies 50A, 50B. These controllers monitor theoutput power consumption and convert the input power to the powerrequired by the computer system 10. Both of the power supplies arecapable of supplying power to the entire computer system. In oneembodiment, the power supplies' voltage regulator circuitry uses a PulseWidth Modulation (PWM) scheme and three-phase regulation with outputfield-effect transistors (FETs).

To better manage power consumption and efficiency, the management entity40, such as the advanced management module of a BLADECENTER serversystem (BLADECENTER is a trademark of IBM Corporation in Armonk, N.Y.),can monitor the power consumption of each server 20. The managemententity 40 runs software, such as IBM POWEREXECUTIVE (IBM andPOWEREXECUTIVE are trademarks of IBM Corporation), that may use theindividual blade server power consumption data to determine the totalamount of power consumed by the computer system 10. Although FIG. 1shows the computer system 10 including only server blades 20, thecomputer system may be defined to include the management entity 40 orany number of other system components, such as an entire rack ofservers.

When the management entity 40 determines that the computer system 10 hascrossed a power consumption setpoint that corresponds with thecross-over efficiency, then the management entity 40 can bring anadditional alternating current to direct current (AC/DC) power supply 50on line. Accordingly, the computer system 10 is supplied with power fromonly one power supply 50 over a first range of power consumption that isless than the power consumption setpoint and is supplied with power fromsynchronous operation of a combination of power supplies over a secondrange of power consumption that is greater than the power consumptionsetpoint. This method of supplying power to the computer system 10combines the greater efficiency of a single heavily loaded power supplyover the first (lower) range of power consumption with the greaterefficiency of multiple synchronous power supplies over a second (higher)range of power consumption.

While the present disclosure provides a detailed discussion of using twopower supplies, it should be understood that the invention may also beapplied to the use of any number of multiple power supplies, such thatthere is a cross-over efficiency between each successively largergrouping of power supplies (i.e., 1, 2, 3, 4, . . . ). However, there isa tradeoff between the greater power efficiencies that multiple powersupplies can provide in accordance with the invention, and the greaterinitial cost of purchasing multiple power supplies.

In one embodiment of the invention, the baseboard management controller(BMC) reads the Vital Product Data (VPD) to determine the powerrequirements for each server blade. This information may be communicatedto the management entity that is monitoring the power consumption of thecomputer system. The management entity may have a power consumptiontable that includes the actual amount of power being consumed by eachserver, and that power consumption table may also indicate the amount orrange of power consumption that is associated with different operationalstates of the server blades. Therefore, when a server blade is to beturned on, this table may be used to determine whether the one or morecurrently active power supplies can provide enough power to allow theserver to be powered on. In addition, the BMC may continuously monitorthe actual power consumption of each server blade and continuouslyupdate the power consumption table. For example, should a server bladego into a sleep state, the BMC for that server blade will communicatethis information to the management entity. Because the management entityis notified of the change in the power state prior to the server bladeactually entering that state, the management can provide a proactivepower management function to more closely manage power efficiency. Thesame statement can be made when the blade server exits the sleep state.Since the management entity is notified of the change in state, themanagement entity can proactively determine if a second power supplyshould be brought on line immediately. Otherwise, the management entityoperates in a reactionary mode.

As server blades are added to the configuration, more power is required.Such a change in the power consumption of the computer system causes achange in the efficiency of the one or more power supply that is turnedon. Each power supply is least efficient at its lowest power output andalso has a lower efficiency at full (100%) power output. Ideally,maximum power efficiency is achieved when the power supply is operatingat approximately 85% of the maximum power rating. When a computer systemrequires additional power, an additional power supply can be brought online to work in combination with a first power supply in order tominimize the reduced power efficiencies associated with low power andfull power operation of power supplies. As additional power supplies arebrought on line, they are brought online before the operational powersupply(ies) reach full power and share the load so that none of thepower supplies operate at very low power.

When the power consumption passes the cross-over efficiency, themanagement entity initializes the second power supply to beginregulation. In one embodiment, the management entity sends a command tothe operating power supply to indicate a change of state and a separatecommand to the second power supply to indicate its operational state.Preferably, an interlock protocol is used to ensure both power supplieshave received the appropriate command, before the management entityissues a synchronization pulse to both power supplies instructing thepower supplies to change their phase regulation to the state indicatedin the command. An example of one method of phase regulation isdescribed in greater detail with regard to FIG. 3.

FIG. 2 is a graph of power efficiency (along the y-axis) as a functionof power consumption (along the x-axis) including a first powerefficiency curve 60 for a single power supply and a second powerefficiency curve 62 for a combination of the first power supplyoperating synchronously with a second power supply that is identical tothe first power supply. In the example of FIG. 2, each of the two powersupplies has a nominal power rating of 2000 Watts. Every 1% increase inpower efficiency at 1000 Watts of total power consumption saves 10Watts, and at 2000 Watts of total power consumption saves 20 Watts. Asthe total power consumption of the computer system increases, anincrease in power efficiency provides ever greater power savings.

In reference to the first power efficiency curve 60 for a single powersupply, it may be noted that the efficiency of the power output beginsto decline at about 85% of the rated maximum power output. The powerefficiency profile for a given power supply is a characteristic of thepower supply design and may be predetermined and stored in either theBMC or the management entity. Alternatively, the power supply could bedesigned to measure its own power efficiency and report that to themanagement entity.

As illustrated by the first power efficiency curve 60, power suppliesgenerally exhibit a similar decline at about 85% of the rated maximumpower output. Accordingly, the AC to DC power conversion efficiency ofthe power supply is generally better at 40% to 50% of the rated outputof the power supply (i.e., about 87% efficiency) than at 90% of therated maximum (i.e., about 86% efficiency). As a result, when thecomputer system is consuming more power than about 90% of the ratedmaximum of a single power supply, the power efficiency can be increased(or a reduction in power efficiency may be avoided) by activating asecond power supply and operating two power supplies to share the loadrather than have only one power supply continue to carry the load.

The point where the first power efficiency curve 60 for a single powersupply intersects the second power efficiency curve 62 is referred to asthe cross-over efficiency 64. In FIG. 2, the cross-over efficiency isabout 87% and occurs at a power consumption of about 1900 Watts.Although it would be ideal to activate the second power supply to workin combination with the first power supply immediately upon the powerconsumption reaching the cross-over efficiency (here, shown at 1900Watts), there is a possibility that small fluctuations in the powerconsumption would cause excessive switching back and forth between onepower supply and two power supplies. This potential for excessiveswitching is avoided in one embodiment of the invention by implementinga first power consumption setpoint 66 (shown at about 2050 Watts) thatwill trigger switching from the first power supply to the combination ofthe first and second power supplies (see arrow 67 illustrating theswitch), and implementing a second power consumption setpoint 68 (shownat about 1800 Watts) that will trigger switching from the combination ofthe first and second power supplies to the first power supply (see arrow69 illustrating the switch). Providing some separation in the values ofthe setpoints 66, 68 prevents excessive switching. As shown, a 250 Wattchange in power consumption is necessary to cause switching. In otherwords, after switching from one power supply to two at 2050 W, themanagement entity would not switch back to only one power supply unlessthe power consumption had decreased to 1800 W. Similarly, afterswitching from two power supplies to one at 1800 W, the managemententity would not switch back to two power supplies unless the powerconsumption had increased to 2050 W.

Other values of the setpoints and the amount of separation between thesetpoints are within the scope of the invention. Still, it is preferablethat the first power consumption setpoint 66 is greater than the powerconsumption at the cross-over efficiency 64 and preferable that thesecond power consumption setpoint 68 is less than the power consumptionat the cross-over efficiency 64 so that the switch always results in anincrease in the power efficiency (i.e., the arrows 67, 69 point upward).However, this preference is not a requirement, but a practical way ofswitching the number of active power supplies to improve the overallpower efficiency. Both setpoints could be greater than the powerconsumption at the cross-over efficiency or both setpoints could be lessthan the power consumption at the cross-over efficiency.

It should also be noted that the setpoints have been described in termsof a power consumption value, but the relationship between powerconsumption and power efficiency means that the setpoints could also bestated in terms of a power efficiency value. Since the invention isdirected at improving the efficiency of the power being supplied to thecomputer system, the power efficiency is perhaps the more relevantvariable to control. However, some additional logic might be required toavoid switching at inappropriate times since even a single power supplycurve may hit a given power efficiency value more than once. For examplethe single power supply curve 60 passes through an 85% efficiency atboth 750 W output and 2050 W output. Switching from one power supply totwo at 750 W would actually cause a significant reduction in powerefficiency. Furthermore, the individual components of the computersystem, such as server blades, already report power consumption to themanagement entity 40, so it may be more convenient and practical toswitch the number of power supplies that are active on the basis ofpower consumption setpoints.

The foregoing methods for switching between the number of power suppliesallows power to be supplied to the computing system using only a firstpower supply over a first range of power consumption 70, and allowspower to be supplied to the computing system using a combination of thefirst power supply and a second power supply over a second range ofpower consumption 72. Near the cross-over efficiency 64, the efficiencyof the first power supply is comparable to the efficiency of thecombination of power supplies. Although the gap between the first range70 and the second range 72 is preferably kept narrow, optionally lessthan 250 W or less than 150 W, the methods of the present invention mayutilize either the first power supply or the combination of powersupplies in the gap.

FIG. 3A is a diagram illustrating power being supplied to a computersystem using a single power supply and FIG. 3B is a diagram illustratingpower being supplied to the computer system using a combination of powersupplies operating synchronously. In FIG. 3A, a first timeline (seriesof time periods T) illustrates the use of only one power supply usingthree-phase regulation. Each of three field-effect transistors (FETs) isactive for 30% of the duty cycle to get a total of 90% of the ratedoutput of the power supply. In FIG. 3B, a second timeline illustratesthe use of two power supplies that each use three-phase regulation, butthe FETs of the two power supplies are synchronized in time. Actually,each of the six FETs in the second timeline (three FETs in each of thetwo power supplies) operate synchronously and are active for 15% of theduty cycle to provide the same amount of power as the single powersupply in FIG. 3A. Accordingly, both of the two power supplies operateat 45% of their rated capacity (i.e., 3 FETs at 15% each), resulting inan increase in the total overall power efficiency.

FIG. 4 is a flowchart of a method 80 of supplying power to a computersystem. The method begins at step 82 and begins monitoring the actualpower consumption of the computer system in step 84. If step 86determines that there is only one active power supply, then the methodbranches to step 88. If step 88 determines that the actual powerconsumption of the computer system is not greater than a first powerconsumption setpoint, then no action is taken and the method returns tostep 84. However, if step 88 determines that the actual powerconsumption is greater than the first power consumption setpoint justabove the cross-over efficiency, then an idle power supply is activatedin step 90 and the two power supplies are operated synchronously in step92 before returning to step 84.

If step 86 determines that there are two active power supplies, then themethod proceeds to step 94 whether the actual power consumption is lessthan a second power consumption setpoint just below the cross-overefficiency. If not, the method returns to step 84 without taking anyaction. However, if step 94 determines that the actual power consumptionis less than the second setpoint, then one of the two power supplies isdeactivated in step 96 leaving one power supply to operate on its own.The method then returns to step 84.

FIG. 5 is a graph of power savings as a function of power consumed in arange near the cross-over in FIG. 2. The graph shows that operating twopower supplies in accordance with present invention results insignificant power savings over a broad range of operation. It should benoted that providing separate first and second power consumptionsetpoints has been disclosed to avoid unnecessary oscillation betweenone and two active power supplies, but this may also lead to a minorregion of negative power savings. This is true where the setpoint doesnot trigger activation of the most efficient number of power suppliesuntil the actual power consumption has already passed the cross-overefficiency. Depending upon the setpoint selection a minor region ofnegative power savings may occur as power consumption rises above thecross-over efficiency, as power consumption falls below the cross-overefficiency, or both. Furthermore, the setpoints and/or the range betweenthe setpoint may be increased or decreased in order to reduce oreliminate “negative power savings.”

As will be appreciated by one skilled in the art, the present inventionmay be embodied as a system, method or computer program product.Accordingly, the present invention may take the form of an entirelyhardware embodiment, an entirely software embodiment (includingfirmware, resident software, micro-code, etc.) or an embodimentcombining software and hardware aspects that may all generally bereferred to herein as a “circuit,” “module” or “system.” Furthermore,the present invention may take the form of a computer program productembodied in any tangible storage medium having computer-usable programcode stored on the storage medium.

Any combination of one or more computer usable or computer readablestorage medium(s) may be utilized. The computer-usable orcomputer-readable storage medium may be, for example but not limited to,an electronic, magnetic, electromagnetic, or semiconductor apparatus ordevice. More specific examples (a non-exhaustive list) of thecomputer-readable medium include: a portable computer diskette, a harddisk, random access memory (RAM), read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), a portablecompact disc read-only memory (CD-ROM), an optical storage device, or amagnetic storage device. The computer-usable or computer-readablestorage medium could even be paper or another suitable medium upon whichthe program is printed, as the program can be electronically capturedvia, for instance, optical scanning of the paper or other medium, thencompiled, interpreted, or otherwise processed in a suitable manner, ifnecessary, and then stored in a computer memory. In the context of thisdocument, a computer-usable or computer-readable storage medium may beany storage medium that can contain or store the program for use by acomputer. Computer usable program code contained on the computer-usablestorage medium may be communicated by a propagated data signal, eitherin baseband or as part of a carrier wave. The computer usable programcode may be transmitted from one storage medium to another storagemedium using any appropriate transmission medium, including but notlimited to wireless, wireline, optical fiber cable, RF, etc.

Computer program code for carrying out operations of the presentinvention may be written in any combination of one or more programminglanguages, including an object oriented programming language such asJava, Smalltalk, C++ or the like and conventional procedural programminglanguages, such as the “C” programming language or similar programminglanguages. The program code may execute entirely on the user's computer,partly on the user's computer, as a stand-alone software package, partlyon the user's computer and partly on a remote computer or entirely onthe remote computer or server. In the latter scenario, the remotecomputer may be connected to the user's computer through any type ofnetwork, including a local area network (LAN) or a wide area network(WAN), or the connection may be made to an external computer (forexample, through the Internet using an Internet Service Provider).

The present invention is described below with reference to flowchartillustrations and/or block diagrams of methods, apparatus (systems) andcomputer program products according to embodiments of the invention. Itwill be understood that each block of the flowchart illustrations and/orblock diagrams, and combinations of blocks in the flowchartillustrations and/or block diagrams, can be implemented by computerprogram instructions. These computer program instructions may beprovided to a processor of a general purpose computer, special purposecomputer, or other programmable data processing apparatus to produce amachine, such that the instructions, which execute via the processor ofthe computer or other programmable data processing apparatus, createmeans for implementing the functions/acts specified in the flowchartand/or block diagram block or blocks.

These computer program instructions may also be stored in acomputer-readable storage medium that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablestorage medium produce an article of manufacture including instructionmeans which implement the function/act specified in the flowchart and/orblock diagram block or blocks.

The computer program instructions may also be loaded onto a computer orother programmable data processing apparatus to cause a series ofoperational steps to be performed on the computer or other programmableapparatus to produce a computer implemented process such that theinstructions which execute on the computer or other programmableapparatus provide processes for implementing the functions/actsspecified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. Each block of the block diagrams and/orflowchart illustration, and combinations of blocks in the block diagramsand/or flowchart illustration, can be implemented by special purposehardware-based systems that perform the specified functions or acts, orcombinations of special purpose hardware and computer instructions.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,components and/or groups, but do not preclude the presence or additionof one or more other features, integers, steps, operations, elements,components, and/or groups thereof. The terms “preferably,” “preferred,”“prefer,” “optionally,” “may,” and similar terms are used to indicatethat an item, condition or step being referred to is an optional (notrequired) feature of the invention.

The corresponding structures, materials, acts, and equivalents of allmeans or steps plus function elements in the claims below are intendedto include any structure, material, or act for performing the functionin combination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but it is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

1. A method for supplying power in a computing system, comprising:monitoring power consumption of the computing system; supplying power tothe computing system using only a first power supply over a first rangeof power consumption; and supplying power to the computing system usinga combination of the first power supply and a second power supply over asecond range of power consumption; wherein the first range is lower thanthe second range, wherein the first power supply provides greaterefficiency than the combination of the first and second power suppliesover the first range of power consumption, and wherein the combinationof the first and second power supplies provides greater efficiency thanthe first power supply over the second range of power consumption; andswitching from the first power supply to the combination of the firstand second power supplies in response to the power consumptionincreasing to a first power consumption setpoint; and switching from thecombination of the first and second power supplies to the first powersupply in response to the power consumption decreasing to a second powerconsumption setpoint, wherein the first power consumption setpoint isgreater than the second power consumption setpoint.
 2. The method ofclaim 1, wherein the combination of the first and second power suppliesoperate in a synchronous mode.
 3. The method of claim 1, wherein thefirst power consumption setpoint is greater than the power consumptioncorresponding to the cross-over efficiency between the first powersupply and the combination of the first and second power supplies. 4.The method of claim 3, wherein the second power consumption setpoint isless than the power consumption corresponding to the cross-overefficiency between the first power supply and the combination of thefirst and second power supplies.
 5. The method of claim 4, wherein thefirst and second power consumption setpoints are no more than 150 Wattsfrom the power consumption at the cross-over efficiency.
 6. The methodof claim 1, wherein the computer system includes a plurality of computerservers, and wherein the power consumption is that amount of powerconsumed by the plurality of computer servers.
 7. The method of claim 6,wherein the step of monitoring the power consumption of the computersystem includes a management entity receiving power consumption datafrom each of the plurality of computer servers.
 8. The method of claim7, wherein each of the plurality of computer servers includes abaseboard management controller that provides server power consumptiondata to the management entity.
 9. The method of claim 1, furthercomprising: changing the number of computer servers in the computersystem.
 10. A computer program product including computer usable programcode embodied on a non-transitory computer usable storage medium formanaging the supply of power in a computer system, the computer programproduct including: computer usable program code for monitoring powerconsumption of the computing system; computer usable program code forinstructing only a first power supply to supply power to the computingsystem over a first range of power consumption; and computer usableprogram code for instructing a combination of the first power supply anda second power supply to supply power to the computing system over asecond range of power consumption; wherein the first range is lower thanthe second range, wherein the first power supply provides greaterefficiency than the combination of the first and second power suppliesover the first range of power consumption, and wherein the combinationof the first and second power supplies provides greater efficiency thanthe first power supply over the second range of power consumption; andcomputer usable program code for switching from the first power supplyto the combination of the first and second power supplies in response tothe power consumption reaching a first power consumption setpoint; andcomputer usable program code for switching from the combination of thefirst and second power supplies to the first power supply in response tothe power consumption reaching a second power consumption setpoint,wherein the first power consumption setpoint is greater than the secondpower consumption setpoint.
 11. The computer program product of claim10, wherein the combination of the first and second power suppliesoperate in a synchronous mode.
 12. The computer program product of claim10, wherein the first power consumption setpoint is greater than thepower consumption corresponding to the cross-over efficiency between thefirst power supply and the combination of the first and second powersupplies.
 13. The computer program product of claim 12, wherein thesecond power consumption setpoint is less than the power consumptioncorresponding to the cross-over efficiency between the first powersupply and the combination of the first and second power supplies. 14.The computer program product of claim 13, wherein the first and secondpower consumption setpoints are no more than 150 Watts from the powerconsumption at the cross-over efficiency.
 15. The computer programproduct of claim 10, wherein the computer system includes a plurality ofcomputer servers, and wherein the power consumption is that amount ofpower consumed by the plurality of computer servers.
 16. The computerprogram product of claim 15, wherein the computer usable program codefor monitoring the power consumption of the computer system includescomputer usable program code for receiving power consumption data fromeach of the plurality of computer servers.
 17. The computer programproduct of claim 16, wherein each of the plurality of computer serversincludes a baseboard management controller that provides server powerconsumption data to the management entity.
 18. The computer programproduct of claim 10, further comprising: computer usable program codefor detecting a change in the number of computer servers in the computersystem.